Wire sawing is the dominant method for generating the thin substrates of semiconductor materials that are generally referred to as “wafers.” Semiconductor wafers are essential to the integrated circuit and photo-voltaic industries, which can more generally be referred to as the solid-state electronics industry. Common substrate materials subjected to “wafering” include silicon, sapphire, silicon carbide, aluminum nitride, tellurium, silica, gallium arsenide, indium phosphide, cadmium sulfide, germanium, zinc sulfide, gray tin, selenium, boron, silver iodide, and indium antimonide, among other materials.
A typical wire sawing process involves drawing a wire across a mass of substrate material or workpiece, which in its unwafered state is commonly referred to as a boule or an ingot. The wire typically comprises one or more of a metal or alloy such as steel. A cutting fluid is applied to the wiresaw in the area where cutting is being conducted. The cutting fluid cools and lubricates the wire and workpiece. A particulate abrasive is typically included in the cutting fluid to increase the efficiency of the cutting. The abrasive is generally selected based on the hardness of the substrate being cut, which harder abrasives being used for harder substrates.
Cutting fluids can be non-aqueous fluids, such as hydrocarbon oils, or other organic materials such as glycols or poly(ethylene glycol) materials. Aqueous fluids, which typically include various amounts and types of organic solvents or polymers dissolved or dispersed in water.
A difficulty in current cutting wire technology is the increasing cost for disposal of spent cutting fluids, and the concern about environmental impacts depending on method chosen for such disposal.
Another difficulty in current cutting wire technology relates to the heat and shear force generated during the course of cutting a workpiece. This heat and shear force arises not only from the friction that is essential for the cutting process at the cutting interface between cutting wire, abrasive particles, and substrate surface, but also from the pumping mechanism and conduit used in delivering the cutting fluid slurry to the cutting interface. The heat and shear force can compromise the integrity of the organic materials present in an aqueous cutting fluid slurry, particularly polymeric materials that are typically added to the fluid to provide a sufficient viscosity to keep the abrasive particles uniformly dispersed and to aid in adhering the cutting fluid to the cutting wire. Break down of polymers having a weight average molecular weight of about 200 kiloDaltons (kDa) or greater is particularly problematic, since such polymers are typically used in relatively small amounts, and reductions in molecular weight due to shear dramatically reduce the viscosity of the cutting fluid. Shear forces occur at the cutting surface as well as inn the pumps and nozzles used to apply the cutting fluid to the wire.
There is an ongoing need for aqueous cutting fluids having relatively high water content, for good cooling ability, while still having sufficient viscosity to maintain abrasive particles as a uniform suspension and adhere the fluid to the cutting wire over many recycle iterations.
The present invention set forth herein below is a useful addition to the field of wiresaw cutting technology.